Recently, a piezoelectric transducer containing crystal or the like as a time source, a timing source for generating control signals and others, or a reference signal source, for example, has been used in a cellular phone and a portable information terminal device. Various types of piezoelectric transducer for this purpose are known, and a surface mount device piezoelectric transducer is an example of these. A typical piezoelectric transducer of this type has three-layer structure constituted by a piezoelectric substrate containing a piezoelectric oscillation piece, a base substrate, and a lid substrate joined with one another in the up-down direction with the piezoelectric substrate sandwiched between the base substrate and the lid substrate. According to this structure, the piezoelectric transducer is accommodated in a cavity (closed chamber) formed between the base substrate and the lid substrate.
Moreover, a two-layer structure type has been developed in more recent years as well as the three-layer structure type discussed above. This piezoelectric transducer has two-layer structure having the base structure and the lid structure directly connected with each other, and the piezoelectric oscillation piece is accommodated in the cavity formed between the two substrates. This two-layer structure type piezoelectric transducer is superior to the three-layer structure type in its smaller thickness and other points, and thus has been used as preferable art.
A tuning-fork-type piezoelectric oscillation piece is known as the piezoelectric oscillation piece accommodated in the cavity. This turning-fork-type piezoelectric oscillation piece has a pair of oscillation arms disposed in parallel, and a base for fixing the base ends of the pair of the oscillation arms as one unit. When predetermined drive voltage is applied to the pair of the oscillation arms, the oscillation arms oscillate in the direction of moving close to each other or away from each other. It is known that a frequency F during oscillation is calculated by F=k(W/L2), assuming that the arm length of each oscillation arm (length in the longitudinal direction) is L and that the arm width is W (in equation, k is a proportional constant).
Generally, a piezoelectric transducer has a determined nominal frequency as a secured frequency during oscillation of a piezoelectric oscillation piece to which predetermined drive voltage is applied. Thus, the frequency of the piezoelectric oscillation piece needs to be controlled in such a manner as to oscillate within the nominal frequency when voltage is applied.
For this purpose, frequency control of the piezoelectric oscillation piece is performed. Generally, the frequency control is divided into two stages of rough control and fine control. The rough control step is conducted during the production process of the piezoelectric oscillation piece as a step for rough frequency control, in which step the frequency comes close to the nominal frequency to some extent. The fine control step is performed after the piezoelectric oscillation piece is sealed into the cavity as a step for fine frequency control, in which step the piezoelectric oscillation piece can finally oscillate within the range of the nominal frequency. The fine control step is particularly an important step which can determine the quality of the piezoelectric oscillation piece.
The fine control step is generally executed by heating a weight metal film formed on the outer surface of the oscillated piezoelectric oscillation piece using laser or the like while measuring the frequency to partially remove the weight metal film (for example, see Patent Reference 1). According to this method, the weight of the piezoelectric oscillation piece is slightly reduced by removing the weight metal film. As a result, the frequency of the piezoelectric oscillation piece increases. Thus, the frequency of the piezoelectric oscillation piece can be gradually raised (adjusted) such that the frequency can approach the nominal frequency.
During manufacture of the piezoelectric transducer, there is also a process for controlling series resonance resistance (R1) by increasing the degree of vacuum inside the cavity as an important process similarly to the frequency control explained above. The series resonance resistance depends on the degree of vacuum inside the cavity. More specifically, the series resonance resistance lowers to approach appropriate resistance until the degree of vacuum inside the cavity reaches a certain level, and the resistance does not greatly vary after the degree of vacuum comes equal to or higher than the constant level. The degree of vacuum inside the cavity is a factor having effect on the frequency of the piezoelectric oscillation piece. Thus, the series resonance resistance needs to be adjusted to appropriate resonance serial resistance before the fine control step.
For example, the series resonance resistance can be controlled by using a getter member made of aluminum or the like inside the cavity as a known method (for example, see Patent Reference 2 and Patent Reference 3). According to this method, the getter member is initially heated by laser or the like for activation. Then, the activated getter member absorbs the air inside the cavity while evaporating. As a result, the degree of vacuum inside the cavity increases, thereby controlling the series resonance resistance. The method of controlling the series resonance resistance by increasing the degree of vacuum is hereinafter referred to as gettering.
Thus, in manufacturing the surface mount device piezoelectric transducer, it is considered that the processes of sealing the piezoelectric oscillation piece into the cavity and then performing gettering and fine control are essential for maintaining the quality of the product.
FIG. 29 is a plan view illustrating a condition of a piezoelectric transducer from which a lid substrate is removed according to related art. FIG. 30 is a cross-sectional view taken along a line D-D in FIG. 29. As illustrated in FIG. 30, a surface mount device piezoelectric transducer 200 which includes a package 209 constituted by a base substrate 201 and a lid substrate 202, and a piezoelectric oscillation piece 203 accommodated in a cavity C formed inside the package 209 has been proposed. A junction film 207 is disposed between the base substrate 201 and the lid substrate 202 to join both the substrates 201 and 202 by anode junction.
It is generally demanded that the piezoelectric transducer has lowest possible equivalent resistance (effective resistance Re). The piezoelectric transducer having low equivalent resistance can oscillate the piezoelectric oscillation piece by low power as a piezoelectric transducer achieving high energy efficiency.
For reducing equivalent resistance, the interior of the cavity C into which the piezoelectric oscillation piece 203 is sealed is brought into a condition close to vacuum as illustrated in FIG. 29 as a typical known method. For bringing the interior of the cavity C close to vacuum, a method of sealing getter members 210 such as aluminum into the cavity C and applying laser to the getter members 220 from outside to activate the getter members 220 (gettering) is known (for example, see Patent References 2 and 3). According to this method, oxygen generated at the time of anode junction can be absorbed by the activated getter members 210, and thus the interior of the cavity C can be brought into a condition close to vacuum.
Patent Reference 1: JP-A-2003-133879
Patent Reference 2: JP-A-2006-86585
Patent Reference 3: JP-T-2007-511102
However, the following problems still arise from the piezoelectric transducer manufacturing method described above.
During initial heating of the weight metal film in the fine control step, the piezoelectric oscillation piece on which the weight metal film is formed is heated as well. In this case, the piezoelectric oscillation piece inevitably receives heating load. Thus, it is desired that the fine control step is performed producing the lowest possible load. However, since variations in the film thickness of the weight metal film are easily produced, a heating load is easily given to the piezoelectric oscillation piece. That is, due to the variations in the film thickness, frequency change differs every time one pulse of laser is applied. In this case, an unexpected and unintended amount of weight metal film is removed due to variations of the film thickness even when the expected amount of the weight metal film is desired to be removed by applying one pulse of laser. As a result, frequency change varies. Thus, laser needs to be repeatedly applied to the piezoelectric oscillation piece in some cases, which may accumulate heating loads on the piezoelectric oscillation piece and give adverse effect on the piezoelectric oscillation piece.
Particularly, since the removed weight metal film cannot be returned to the original state, the frequency increased once cannot be returned to a lower frequency. Thus, removal of the weight metal film needs to be carefully carried out little by little, which easily accumulates heating loads on the piezoelectric oscillation piece and gives adverse effect on the piezoelectric oscillation piece with high possibility.
Moreover, the steps of gettering and fine control essential to maintenance of the quality are separately performed at different times. In this case, a long time is required to manufacture the piezoelectric transducer, which lowers working efficiency.
The invention has been developed considering these circumstances. It is an object of the invention to provide a surface mount device piezoelectric transducer capable of performing fine frequency control with higher accuracy while reducing accumulation of heating loads as much as possible and performing gettering and fine control with high efficiency. It is another object of the invention to provide a piezoelectric transducer manufacturing method for manufacturing the piezoelectric transducer, an oscillator, an electronic device, and a radio clock each of which includes the piezoelectric transducer.
For controlling the frequency of the piezoelectric transducer 200 including the tuning-fork-type piezoelectric oscillation piece 203 shown in FIG. 29, such a method is known which attaches metal weight members 211 for frequency control to the tips of the oscillation arms 210 of the piezoelectric oscillation piece 203 and partially fuses and removes the weight members 211 by applying laser beam for trimming such that the mass of the weight members 211 can be reduced (for example, see Patent Reference 1).
According to the related-art frequency control method, the mass of the oscillation arms 210 is reduced by trimming the weight members 211. Thus, frequency control can only increase the frequency of the piezoelectric transducer 200. In this case, when the frequency of the piezoelectric transducer 200 exceeds a target value by excessively trimming the weight members 211, the piezoelectric transducer 200 needs to be thrown away for the absence of the method of decreasing the frequency to the target value. Accordingly, the yield of the piezoelectric transducer 200 unfortunately lowers.